The Plagiarism Tutorial has concise definitions of plagiarism and citation; 3 pages that explain and illustrate acceptable and unacceptable paraphrasing of research; and quizzes for students (U of Southern MI Libraries).

Common knowledge, which does not need to be cited, is more difficult to define in the digital age. Harvard has a definition here to which you could refer students.

Editing: Richard A. Lanham’s “paramedic method” is a great teaching and revision tool. The method includes 7 steps for “diagnosing” convoluted writing that will help students craft more direct and concise sentences. (See Revising Prose 5th ed. for full explanation.)